Adhesive composition derived from hydrolyzed ethylene-vinyl acetate interpolymers



Patented Aug. 20, 1946 ADHESIVE. COMPOSITION DERIVED FROM HYDROLYZED ETHYLENE-VINYL ACETATE- INTERBOLYMERS William H. sharkemwilmington, Henry M. Cadot, Greenville, and William B. Clark, Wilmington, Del., assignors' to E. I. du Pont de Nemours & Company. Wilmington, Del'., a corporation of Delaware No Drawing. Application May 12, 1944,

Serial No. 535.380 f 6 Claims. (01. 20-89) This invention relates to a process for adhesively bonding one surface to another; It also relates to plywood, and other laminations which are united by means of an adhesive. More particularly, it relates to plywood in which the plies are umted by means of an adhesive bond comprising a reaction product of an aldehyde with a hydrolyzed-ethylene-vinyl ester interpolymer.

Hitherto, plywood adhesives have contained various glue-like or resinous products, including phenol-formaldehyde and urea-formaldehyde resins. Relatively few of these adhesives were useful in manufacturing plywood having good resistance to hot water. Some. of the best commercial plywood adhesives which gave bonds having good hot water resistance had other properthem'to the action of heat. The formaldehydereactive substances include not only formaldehyde itself, but also materials which contain formaldehyde in reactive form.

- The hydrolyzed ethylene interpolymers which may be. used in accordance with this invention,

may be prepared, if desired,by themethod described in the copending application S. N. 446,114

of J. R. Roland, filed June 6, 1942. The preferred hydrolyzed interpolymers are the interpolymers of ethylene with vinyl esters of monocarboxylic ties which were undesirable, such as instability on long storage. Moreover, when such adhesives were applied in film form in the manufacture of plywood, excessively high temperatures and pressures generally had to be used to produc a strong thermally hardened bond, and the films themselves were brittle at low temperatures, and tacky at elevated temperatures. Still another limitation of the heretofore available plywood adhesives was that they were unsatisfactory except when the moisture content of the wood was carefully controlled. Thus certain adhesives, for best results. had to be used with wood having a moisture content of 8% to 12%, while others required wood having or less of moisture for -best results.

acids, especially vinyl acetate, propionate, and the like. I The compositions containing formaldehydereactive substances and hydrolyzed interpolymers of ethylene and vinyl esters include'the resins obtained by condensation of hydrolyzed ethylenevinyl ester interpolymers with formaldehyde and such formaldehyde-reactive substances as paraformaldehyde and the alkyl or substituted alkyl acetals; N-methylol compounds and their ethers, including dimethylol urea, N-alkoxyalkylureas, N-methylol compounds of melamine or of substitutedaminotr'iazines and ethers: thereof, N-

methylol'and N-alkoxymethyl amides and amines including N-methyloi acetamide, N-methylol formamide, N-alkoxymethyl nylons, and N-alk- 'oxymethyl phthalimides; uron, and methylene dim-ea; methylol phenols including phenol-form- An object of this invention is to provide improved plywood compositions. Anotheroblect is to obtain plywood compositions having excellent dry and wet shear strengthsQeven when relatively small weights of adhesive per unit of glue line area are employed. Another object isto providether object is to provide a, lywood adhesive'which is highly effective with wood of various moisture contents. Still another object is to provide a novel method for uniting surfaces.

aldehyde condensationproducts; cyclic formals,

such as dioxolane and trioxane; hexamethylene tetramine, and its salts; and methylene glycol esters Included also are the formaldehydetreated hydrolyzed ethylene-vinyl ester interpolymers disclosed in copending application S. N. 447,589 of W. H. Sharkey. filed June 18, 1942, and the condensation products of hydrolyzed ethylene-vinyl ester interpolymers with N-methylol and N-alkoxymethyl compounds disclosed in application S. N. 469,460, of D. C. Pease, filed December 18, 1942.

These and other objects are accomplished, in

' catalyst, and (2) maintaining the surfaces in contact with the said composition. while subjecting f The products which are obtained by treating formaldehyde with hydrolized ethylene-vinyl ester interpolymers in the' presence of an acidic catalyst at low temperature (below about C.) usually are soluble, and can be softened by heat. These products may be thermally hardened, preferably by heating to temperatures above "100" C., and the resultant resins are relatively insoluble. Generally, the lattertype of formaldehyde-treated hydrolyzed interpolymer is either high-melting or fails to melt below the decomposition temperature. The preferred adhesive bonds, in accordance with this invention. contain I formaldehyde.

thermally hardened formaldehyde-treated 'hy-f drolyzed interpoiymer.

Among the preferred adhesives of this invention are the condensation products of formaldelrvde with hydrolyzed ethylene-vinyl-acetate interpolymers. In the preparation of these products. best results are obtained when the hydrolyzed ethylene-vinyl acetate interpolymer has an intrinsic viscosity (defined by. Kraemer, Ind. Eng. Chem, 38, 1201 (1938), as being the natural logarithm of the relative viscosity of a dilute aldehyde content in the formaldehyde-treated resin permits the manufacture of plywood havin remarkably high shear strength This is shown in the following table. The tests herein described were made with standard 3-ply birch plywood,

'havinga panel thickness of ca. 0.185 inch.

Table I.-E'1fect of formaldehyde content of the solution, divided by the concentration, relative viscosity being the viscosity of the solution divided by the viscosity of the solventl' of at least 0.1,

and preferably about 0.7 to 1.3. The ethylene to vinyl acetate ratio in the interpolymer priorto hydrolysis may vary very widely, but best results are obtained when this ratio isbetween 1.0 and 10. When this ratio is low (1.0 or lower) the products are water sensitive, and when it ishigh or higher) the products do not yield adhesives having optimum bond strengths. Excellent resultsare obtained when the ratio lies between 2.0 and 5.0, and the intrinsic viscosity ofth hydrolyzed interpolymeri is within the .preferred range noted above. When the intrinsic viscosity is low (about 0.1 to 0.5) it is preferable that the ethylene-vinyl acetate ratio be relatively high (above about 4).

It is not essential that the polymer be completely hydrolyzed, although the resin films tend to be tacky if the hydrolysis is less than about 75% complete. However, these tacky resins, when treated with, formaldehyde, yieldthermosetting adhesives ofexcellent quality. Thus, a tacky film cast from a 40% hydrolyzed ethylene-vinyl acetate polymer gave, when hardened with formaldehyde, -a bond which-had thigh wet' and. dry strength. Generally, and especially in the manufacture of film type-adhesives, it is desirable to obtain a non-tacky hydrolyzate, and this is ac- The reaction with formaldehyde isconducted preferably by mixing the hydrolyzed interpolymer with formalin in a mutual solvent, such as diox-.

ane, methanol-chloroform mixture, dimethyl formamide or pyridine, in the presence of a small amount of organic acid catalystat a temperature of about 60 to 80 C. Ready-to-press films, which are non-tacky and stable are obtained by evaporation of solvent from the resultant solution. These films according to this invention m y be used as thermosetting interlayers in plywood manufacture, andin this manner plywood or excellent qualityis obtained.

The thermosetting films or this invention are suitable for use in making either molded" or filat-press'f plywood and also for bonding other laminating materials. A convenient temperature for hardening the adhesive of this invention in the making of plywood is about 100 to 150 0.. preferably 120 to 125 C. Generally the bond, in

"accordance with this invention, is hardened at a pressure of about 60 to 200- pounds per square inch. Excellent results are obtained when the bond is'hardefied at a temperature of 120 to 125 C. and a pressure of about'ls pounds .per square inch or higher. h rdening temperature as high as 200' c. or even higher maybe emplo ed when the-materials to be laminated are not adversely affected by such high temperatures.

One of the surprising features or this invention is the discovery that a critical range of f rmsirengthup to 1,000 pounds per square ccmplished easily by carrying the hydrolysis nearv ly to. completion, rior toreactingthe resin with resin on birch plywood shear strength. (ethylene-vinyl acetate ratio, 2.3; intrinsic inscosity of hydrolyzed ethylene-vinyl acetate interpolymer, 0.93; shear strengths determined according .to Army Navy Specification 'AN-NN-P-Sllb.)

I This sample had my shear strength too high for determination in a standard plywood testing rnaehiurlie which measuresshear 'I'he'above table shows thatwhen the weightof HCHOiswithln the range of about 13 to 20% of th total resin weight, (ethylene-vinyl acetate being 2.3) plywood of outstanding quality may be prepared. 3

Further study of this surprising phenomenon showed thatthe critical range of formaldehyde.

content varied with the ratio of ethylene-vinyl acetate in the resin which was used as startin material. when the ethylene-vinyl acetate ratio was 5, the optimum formaldehyde content of the resin was about 10%, and the panels obtainedunder such conditions were of outstanding quality (dry shear strength, 960 pounds per square inch, wet'shear strength 505 pounds per square inch). when the ethylene-vinyl acetate ratio was 10, the plywood had optimum strength lithe HCHO content of theresin was about 2.3 to 4.0%. Higher percentage of HCHO gave panels having lower 'strength'when the ethylene-vinyl acetate ratio was 10 or higher. These'data seemed to indicate a maximum adhesive bond strength at a formaldehyde content corresponding to about 0.5 mole of HCHO per equivalent of hydroxyl in the hydrolyzed interpolymer. The outstandingly strong adhesive bonds were obtained. as shown by these data, when formaldehyde content of the adhesive corresponded to about 0.4 to 0.55 moles per equivalent of hydroxyl in the hydrolyzed interpolymer.

In the tests recorded in Table I, the weight of resin applied was about 25 pounds per 1000 square feet of area at theglue line. Since Army-Navy Specification AN-NN-l?-511b for aircraft plywood requires at present a minimum shear strength of 380 pounds per square inch dry, and 290 pounds per square inch wet, it is clear that these samples were so far above minimum requirements,

"that a smaller weight of adhesive could safely be employed. This is shown in greater detail in the following table.

Table IL-Efiect of quantity of adhesive solids on birch plywood shear strength. (Ethylenevinyl acetate ratio=2.3; intrinsic viscosity of hydrolyzed ethylene-vinyl acetate interpolymer, 0.93; HC'HO in resin, to

The data presented in Table I! show that plywood of acceptable quality may be prepared when as little as 6 pounds of resin is used for 1000 square feet of area at the glue line. This represents a significant advance in the plywood art, particularly as applied to the manufacture of aircraft plywood, since the weight of hitherto available commercial adhesives commonly employed was about 12 to pounds per 1000 square feet of area at the glue line.

While it is preferred to apply the adhesives of the present invention in solid film form it is also possible to apply them in any other convenient form such as a powder, solution, dispersion or slurry. The adhesive may also be applied in the form of impregnated paper, or cloth, or sheeting comprising cloth bound to paper. The solid resin films of the present invention, however, are much stronger and much less brittle than the films of other plywood adhesives which require reinforcement by paper or cloth, and hence the resin films of this invention may be applied directly to the wood. The preferred adhesive films herein disclosed are remarkably stable, and may bestored for many months without appreciable change. They are neither tacky nor stiff over a wide range of temperature (about l0 C. to somewhat above +60 C.) and hence may be shipped or stored in the form of a roll from which the adhesive may be unwound as needed. This is of considerable convenience in the large scale production of plywood. r

The invention is illustrated further by the following examples.

Example I.--Into a 3-necked flask equipped with a stirrer and reflux condenser were placed 60 grams of completely hydrolyzed ethylene-vinyl acetate interpolymer (ratio of ethylene :vinyl acetate, 2.3; intrinsic viscosity ofhydrolyz'ed interpolymer, 0.93) and 240 grams of pyridine. This mixture was heated at 120 with stirring until solution was complete. It was then allowed to cool down, and 0.6 grams of maleic acid was added. To '75 grams of the resulting solution was added 6.6 grams of 37% aqueous formaldehyde solution at 60 to 80 0., and the mixture was agitated until uniform. A film was then cast from the solution on a glass plate with the aid of a doctor blade. The film which resulted upon evaporation of the solvent was flexible and non-tacky over the temperature range from about -10 C. to +60 C. A standard plywood panel, bended at 120 to 125 C. under 200 pounds per square inch line (25 pounds per 1000 square feet glue line), had a dry strength of more than 1000 pounds per square inch, and a wet strength of 565 pounds per square inch with woodfailure.

Example II.-To grams of a pyridine solution containing 30 grams of hydrolyzed ethylenevinyl acetate interpolymer (ratio of ethylenezvinyl acetate, 2.351; intrinsic viscosity of hydrolyzed interpolymer, 0.93) and 0.3 gram of maleic acid was added 5 grams of butylated urea-formaldehyde resin (60% solids) and the mixture was agitated until uniform. The warm solution was spread with a brush on eachside of a standard core ply, and on the core side of .two standard face plies. The plies were dried overnight. Next day the plywood was bonded at to C. under a pressure of 200 pounds per square inch. It had a dry strength of 870 pounds per square inch (90% wood failure). and a wet strength of 390 pounds per square inch (no wood failure).

Example III.Example I was repeated using the aldehyde-reactive compounds stated in the following table, in place of aqueous formaldehyde.

Table IIl.Eflect of various aldehyde-reactive agents on hydrolyzed ethylene-vinyl acetate interpolymer adhesive. (Standard birch plywood panels, 25 pounds adhesive solids per 1000 square feet of glue line area; ethylene; vinyl acetate ratio, 2.3; intrinsic viscosity of hydro- Example IV. 1A slurry was prepared by mixing 20- grams of powdered hydrolyzed ethylene-vinyl acetate interpolymer (ethylenewinyl acetate= 3.3; intrinsic viscosity=0.47) with 35 grams of ethanol and 40 grams of water. To the mixture was added 5.0 grams of butylated urea-formaldehyde (60% solids), and the resulting paste was stirred untiluniform. Direct application of the paste to the plies as described in Example If (25 pounds per 1000 square feet glue line) followed by I bonding the plies at to C. under 200 pounds per square inch pressure gave a plywood panel which had a dry strength of 830 pounds per square inch, but which delaminated when placed in boiling water,

Example V.A solution containing 80 grams hydrolyzed ethylene-vinyl acetate interpolymer (ethylenezvinyl acetate=2.3, intrinsic viscosity: 0.93) and 200 grams of butylated ureaformaldehyde resin in 106 grams of isobutanol and 214 grams of trlchloroetlwlene was admixed in a mechanical disperser with 40 grams of water and 10 grams of partially hydrolyzed polyvinyl acetate (commercial grade RH-488, manufactured by the Du Pont Company) The resulting dispersion was applied to a standard birch plywood test panel as described in Example II (25 pounds of solids per 1000 square feet of glue line), and the resulting plywood, bonded at 120 to 125 under 200 pounds per square inch pressure for 20 minutes, had a for 20 minutes with one layer of film for each glue 75 dry strength of 995 pounds per square inch (60" wood failure) and a wet strength oi 300 pounds Table IV.E17ect of moisture content of wood on adhesive bond strength (Standard plywood panels, 14 pounds adhesive solids per 100 square feet of glue line area; ethylene-vinyl acetate ratio, 2.3; intrinsic viscosity of hydrolyzed tnterpolymer, 0.93)

Moisture Plywood shear strength, pounds per square inch content oi the wood,

per cent Dry Wet 890 (1007 wood failure 560 i wood iaiiure 920 100 wood iailure B45 wood failure 720 100 ,wood failure 500 wood failure It will be understood that the present invention,

- or not; no additional catalyst is essential. The

catalyst may be derived from a substance which generates acid on heating. It is advantageous to have the pH of the adhesive within the range of about 4 to 6 during the thermosetting operation, and this may be achieved by th us of conventional butters, if desired. However, adhesives of the present invention have good storage properties even without buflering the catalysts. Numerous 5 salts, including metal halides, acidic salts, and the like, may be employed as catalysts if desired. Extenders, such as wood flour may be employed with the adhesive composition, without excessive loss of dry bond strength.

Since many apparently widely difierent embodiments of this invention may be made without departing from the spirit and scope thereof, we do not limit ourselves, except as set forth in the following claims.

We claim:

1. Plywood adhesively bonded by an adhesive comprising a carboxylic acid catalyst and a condensation product of formaldehyde with a hydro lyzed ethylene-vinyl acetate interpolymer,

2. An article comprising two cooperating members adhesively united with a condensation product of a hydrolyzed interpolymeroiethylene and a vinyl ester of an organi mono-basic carboxylic acid and formaldehyde.

3. A solid film formed by condensing hydrolyzed ethylene-vinyl acetate ihterpolymer with formaldehyde, the quantity of formaldehyde being.

about 0.4 to 0.55 mole per equivalent of hydroxyl in the hydrolyzed inter-polymer, said fllmcontaining an acid-reacting catalyst and being characterized in that it is non-tacky stable on prolonged storage, and thermally reactive when used as a plywood adhesive interlayer.

4. The article set forth in claim 1 in which the said acid catalyst is maleic acid.

5. The article set forth in claim 1 in which the said interpolymer has an ethylenezvinyl acetate mole ratio of from 1:1 to 10:1.

6. The article set forth in claim 1 in which the formaldehyde content of the adhesive is 0.4 to 0.55 mole per equivalent of hydroxyl in the hydrolyzed interpolymer.

7. The article set forth in claim 1 in which the said interpolymer has an ethylenezviny1 acetate mole ratio of from 2.0 to 5.0 and an intrinsic viscosity of about 0.7 to 1.3.

- WILLIAM H. SHARKEY.

HENRY M. CADOT.

WILLIAM B, CLARK. 

